Of critical importance to the function of the laboratory is the preparation of highly purified complement components with high specific activity. A new small scale procedure for the purification of C3 was developed which yields a fully active, homogeneous protein within three days with recovery of over 70%. We have also shown that C4, C5 and C9 as well are obtainable by this protocol. A rapid method for the purification of homogeneous fully active C1 Inhibitor has also been developed. Use of this inhibitor has allowed Drs. Tenner and Frank to demonstrate that on cell surfaces C4 prevents C1 Inhibitor from interacting with C1. C2 has been isolated by a recently developed, rapid, 3 step procedure involving PEG precipitation, DEAE-Sephacel chromatography and functional affinity chromatography on C4b-Sepharose. The method only needs quantitative analysis of recovery and functional purity for completion. A new rapid procedure for the purification of human C5a suitable for use in human subjects was developed. Extension of the new procedure has allowed for the isolation of C3a as well.